Hughes Alec, Huang Yuexi, Pulkkinen Aki, Schwartz Michael L, Lozano Andres M, Hynynen Kullervo
Department of Medical Biophysics, University of Toronto, Toronto, Canada. Physical Sciences Platform, Sunnybrook Research Institute, Toronto, Canada.
Phys Med Biol. 2016 Nov 21;61(22):8025-8043. doi: 10.1088/0031-9155/61/22/8025. Epub 2016 Oct 25.
Recent clinical data showing thermal lesions from treatments of essential tremor using MR-guided transcranial focused ultrasound shows that in many cases the focus is oblique to the main axis of the phased array. The potential for this obliquity to extend the focus into lateral regions of the brain has led to speculation as to the cause of the oblique focus, and whether it is possible to realign the focus. Numerical simulations were performed on clinical export data to analyze the causes of the oblique focus and determine methods for its correction. It was found that the focal obliquity could be replicated with the numerical simulations to within [Formula: see text] of the clinical cases. It was then found that a major cause of the focal obliquity was the presence of sidelobes, caused by an unequal deposition of power from the different transducer elements in the array at the focus. In addition, it was found that a 65% reduction in focal obliquity was possible using phase and amplitude corrections. Potential drawbacks include the higher levels of skull heating required when modifying the distribution of power among the transducer elements, and the difficulty at present in obtaining ideal phase corrections from CT information alone. These techniques for the reduction of focal obliquity can be applied to other applications of transcranial focused ultrasound involving lower total energy deposition, such as blood-brain barrier opening, where the issue of skull heating is minimal.
近期临床数据显示，使用磁共振引导的经颅聚焦超声治疗特发性震颤产生的热损伤表明，在许多情况下，焦点相对于相控阵的主轴是倾斜的。这种倾斜有可能将焦点扩展到脑的外侧区域，这引发了人们对倾斜焦点成因的猜测，以及是否有可能重新调整焦点的思考。对临床导出数据进行了数值模拟，以分析倾斜焦点的成因并确定校正方法。结果发现，通过数值模拟能够将焦点倾斜度复制到与临床病例相差[公式：见原文]的范围内。随后发现，焦点倾斜的一个主要原因是旁瓣的存在，这是由阵列中不同换能器元件在焦点处功率分布不均所致。此外，还发现通过相位和幅度校正可使焦点倾斜度降低65%。潜在的缺点包括在改变换能器元件之间的功率分布时需要更高水平的颅骨加热，以及目前仅从CT信息获取理想相位校正存在困难。这些减少焦点倾斜度的技术可应用于经颅聚焦超声的其他涉及较低总能量沉积的应用，如血脑屏障开放，在这种情况下颅骨加热问题最小。
